chore(perf.rs): Use PathBuf::join instead of PathBuf::push.

Author: Sol-Ell

src/bootstrap/src/core/build_steps/perf.rs

@@ -161,9 +161,7 @@ Consider setting `rust.debuginfo-level = 1` in `bootstrap.toml`."#);
     }
 
     let sysroot = builder.ensure(Sysroot::new(compiler));
-    let mut rustc = sysroot.clone();
-    rustc.push("bin");
-    rustc.push("rustc");
+    let mut rustc = sysroot.join("bin/rustc");
     rustc.set_extension(EXE_EXTENSION);
 
     let rustc_perf_dir = builder.build.tempdir().join("rustc-perf");